Basics

// Primitives
let name: string = "Ada";
let age: number = 30;
let active: boolean = true;

// Arrays
let items: string[] = ["a", "b"];
let numbers: Array<number> = [1, 2, 3];

// Tuples
let pair: [string, number] = ["Ada", 30];

// Union
let id: string | number = "abc";
id = 123;

// Literal
let status: "active" | "inactive" = "active";

// Any & Unknown
let data: any = "anything";      // escape hatch
let safe: unknown = "safer";     // type-safe any

interface Person {
  name: string;
  birthYear: number;
  readonly id: number;
  email?: string;              // optional
}

const p: Person = { name: "Ada", birthYear: 1985, id: 1 };

interface Employee extends Person {
  company: string;
}

type Status = "active" | "inactive";
type ID = string | number;

type Point = {
  x: number;
  y: number;
};

function greet(name: string): string {
  return `Hello, ${name}`;
}

// Arrow
const add = (a: number, b: number): number => a + b;

// Optional & default params
function build(name: string, active: boolean = true): void { ... }

// Overloads
function parse(input: string): number;
function parse(input: number): string;
function parse(input: string | number): string | number { ... }

function first<T>(items: T[]): T | undefined {
  return items[0];
}

first([1, 2, 3]);      // number
first(["a", "b"]);     // string